Thermal Neutron

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thermal neutron

[′thər·məl ′nü‚trän]
One of a collection of neutrons whose energy distribution is identical with or similar to the Maxwellian distribution in the material in which they are found; the average kinetic energy of such neutrons at room temperature is about 0.025 electronvolt. Also known as slow neutron.

Thermal Neutron


a neutron having a kinetic energy of between 5 × 10–3 and 0.5 electron volt (eV). Such neutrons are called thermal because they are obtained when neutrons are moderated, or slowed down, to thermal equilibrium with the atoms of the moderating medium; the process of reducing the energy of a neutron to the thermal region is called thermalization. In English the term “slow neutron” is often used synonymously with “thermal neutron.” It should be noted, however, that “slow neutron” is also used, particularly in the Soviet literature, in a broader sense (seeNEUTRONS, SLOW).

The velocity distribution of thermal neutrons in a moderator is determined by the moderator’s temperature in accordance with the Maxwellian distribution. The energy corresponding to the most probable velocity of thermal neutrons is 8.6 × 10–5T eV, where T is the absolute temperature in degrees Kelvin. The average kinetic energy of thermal neutrons at room temperature is 0.025 eV, which corresponds to a velocity of 2,200 m/sec and a de Broglie wavelength λ of 1.8 angstroms (seeNEUTRON OPTICS).

Since λ is close to the interatomic distances in solids, the structure of solids can be studied by means of the diffraction of thermal neutrons. The neutron’s possession of a magnetic moment means that the magnetic structure of solids can be investigated through coherent magnetic scattering of thermal neutrons. Since the changes in energy occurring in the inelastic scattering of thermal neutrons in condensed media are comparable with the neutrons’ original energies, the motion of atoms and molecules in solids and liquids can be studied by means of the inelastic scattering of thermal neutrons (seeNEUTRON DIFFRACTION ANALYSIS). Thermal neutrons are of immense importance in the operation of nuclear reactors, since such neutrons are used to produce the chain reaction involving the fission of U and Pu. An important role is also played by thermal neutrons in the production of radioactive isotopes.


References in periodicals archive ?
5 clearly demonstrate to the reader the complex nuclear-physical processes taking place in the active zone of a water-water nuclear reactor operating on thermal neutrons. In these processes, the formation of powerful neutron fluxes and fast charged microparticles, which cause intensive Vavilov-Cerenkov radiation in the water coolant of the reactor, takes place in the reactor core.
(125),(126) As an alternative to [sup.3]He gas, many efforts have been made to develop scintillators containing [sup.6]Li and [sup.10]B, which have high interaction probabilities against thermal neutrons. Before the 3He problem, the applications of scintillators for neutron detectors were not many, and this is a new subject of scintillator research.
These materials are characterized by high thermal neutron absorption cross sections and are almost transparent to fast neutrons.
The effective tumor ablation by Gd-DO3A-BTA could therefore be associated with the high accumulation of Gd in tumor tissue, which in turn captures sufficient thermal neutrons to kill the tumor.
The larger gain setting of the amplifier was used to record low energy gamma rays with energies below 2.22 MeV by adjusting the hydrogen peak location near the end channel of the spectrum, as shown in Figure 6 which shows the low energy activation spectrum of the Ce[Br.sub.3] detector due to the capture of thermal neutrons in the Ce[Br.sub.3] detector material.
Formally, the difference in the height of the main peak (or surface area) thermoluminescent curves for detectors with isotopes [sup.6]Li and [sup.7]Li, placed in the same measuring point, resulting from the differences in cross sections for [sup.7]Li and [sup.6]Li on the thermal neutron absorption, allows for the calculation of the contribution of thermal neutrons to the total TLD signal in that measurement point.
Thermal neutrons emitted from a radioactive substance interact with the nuclei of boron atoms within boron nitride.
thermal neutrons, are highly efficient in causing fission (splitting of
An evaluation of effects of mass selection and seed irradiation with thermal neutrons on yields of corn.
The parameter [[epsilon].sub.0] gives the ratio of detected alphas/tritons to incident thermal neutrons. It was calculated using
In other words, the fast neutrons have to travel a greater distance to become thermalised, and consequently the number of thermal neutrons returned to the detector is reduced.
Thermal neutrons have wavelengths that match the distance scales found in condensed matter systems (ie.